The Gers sealed a 9-5 aggregate victory over the Belgian outfit with a 5-2 win on the night, and Morelos put in a eye-catching performance throughout.
It took the Colombian just nine minutes to open the scoring at Ibrox, and he went to provide further assists for Ryan Kent and Nathan Patterson.
The 24-year-old’s strike was his 13th of the season in all competitions, and speaking after the match, Steven Gerrard hailed his forward for an “unplayable” performance.
Reflecting on the game during an appearance on Sky Sports’ Football Show, former Celtic hitman Hartson said: “They’ve a fantastic threat up top in Alfredo Morelos.
"He divides opinion, he’s a little bit unpredictable, but he’s a nuisance and he gets you goals.
"He’s the type of player who backs in, he’s strong. He'd be a nightmare to play against.
"Steven Gerrard is right, he was unplayable last night.”
Ex-Ibrox striker Kris Boyd echoed Hartson’s sentiments later in the day, telling Sky Sports: “We saw last night, Alfredo Morelos was back to his best.
"I thought his hold-up play was excellent.
"He was involved in eight of the nine goals that Rangers scored over the ties, so overall, in the two games, he had a massive say in it”.
Gerrard’s men have been drawn against Slavia Prague in the final 16 of the Europa League.
The Czech side beat Brendan Rodgers’ Premier League high-flyers Leicester City in the last round of the competition.